Abstract
The proportion of anoestrous ewes ovulating after exposure to a sexually active ram is variable mainly due to whether an LH surge is induced. The aim of this study was to determine the role of oestradiol (E2) in the ram-induced LH surge. In one study, we measured the plasma concentrations of E2 in ewes of different breeds before and after the 'ram effect' and related these patterns to the presence and latency of the LH surge, while another compared ovarian responses with the 'ram effect' following exposure to rams for 2 or 12 h. In all ewes, the concentration of E2 increased 2-4 h after rams were introduced and remained elevated for 14.5 ± 0.86 h. The quantity of E2 secreted before the LH surge varied among breeds as did the mean concentration of E2. The granulosa cells of IF ewes collected after 12 h exposure to rams secreted more E2 and progesterone and had higher levels of StAR than the 2 h group but in MV ewes there was no differences between these groups for any of these parameters. These results demonstrate that the LH surge induced by the rams is a result of increased E2 secretion associated with increased levels of STAR in granulosa cells and that these responses varied among breeds. The results suggest that the variable occurrence of a LH surge and ovulation may be the result of variable ovarian responses to the 'ram effect' and insensitivity of the hypothalamus to the E2-positive feedback signal.
Highlights
In sheep, goats and many wild ungulates, reproduction is seasonal and the females give birth when the environmental conditions are most favourable for the survival of their young
These results demonstrate that the luteinising hormone (LH) surge induced by the rams is a result of increased E2 secretion associated with increased levels of STAR in granulosa cells and that these responses varied among breeds
In 65% of the ewes with an LH surge, there was a continuous pattern of increased E2 (Fig. 1: IF, no. 14; R, no. 1 and M, nos 33 and 7) whereas in the other 35% had a discontinuous pattern of increased E2 (Fig. 1: IF, no. 1 and MV, no. 4)
Summary
Goats and many wild ungulates, reproduction is seasonal and the females give birth when the environmental conditions are most favourable for the survival of their young. The introduction of a sexually active male into a group of seasonally anoestrous ewes will induce an immediate increase in the pulsatile secretion of LH (short-term LH response) This socio-sexual stimulation of ovulation often referred to as the ‘ram effect’, initiates a sequence of q 2015 Society for Reproduction and Fertility ISSN 1470–1626 (paper) 1741–7899 (online) physiological events that in some of ewes culminates in a LH surge and ovulation (Martin et al 1986, Ungerfeld 2007). A similar phenomenon occurs in goats and in many wild ungulates in which shortening and concentrating the period of sexual activity reduce the risks of predation In farm animals, this is a simple technique for inducing fertile mating outside of the breeding season in a way that is compatible with sustainable, green and ethical agriculture (Martin et al 2004).
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